Multilayered animals

Worms

Although all worms have more than two layers of cells and most have long slender bodies, the various groups of worms are different from each other in other respects.

The simplest worms are the flatworms (phylum Platyhelminthes), most of which have flattened shapes like leaves or ribbons. Although musculoepithelial cells have been found in some flatworms, the muscle cells in most are distinct from the epithelial cells. There is a layer of circular muscle fibres immediately under the epidermis, a layer of diagonal fibres, and a still deeper longitudinal layer. There are also dorsoventral muscle fibres running from the upper to the lower epidermis of the flattened body. These sets of muscle fibres act in various combinations to make the body long and thin, short and fat, or bent to one side or the other. These muscles are also used by some of the larger flatworms to pass waves of muscular contraction along the body, enabling the worm to crawl in a snail-like fashion.

Many flatworms have a mouth opening connected to the pharynx, a muscular tube that carries food from the mouth to the intestine. In some flatworms the pharynx is protruded and inserted into invertebrate prey, to digest and suck out the contents. The sucking is done by peristalsis, waves of muscular contraction that move along the tube from the mouth toward the gut. Although the muscle cells of flatworms are generally not musculoepithelial, their nuclei are found in large cell bodies. The muscle fibres of vertebrates and higher invertebrates, on the other hand, have no projecting cell body.

Roundworms (phylum Nematoda) also have large cell bodies on their muscle cells, but these muscle cells are unique in that nerve fibres do not travel to them as they do in the muscles of other animals. Instead, narrow projections of the muscle cell bodies extend to the principal nerves and contact nerve cells there.

Mollusks

The phylum Mollusca includes the gastropods (snails, slugs, and periwinkles), bivalves (clams, oysters, mussels, and scallops), cephalopods (octopods and squids), and other, smaller classes. All mollusks, except the cephalopods, have a highly muscular organ called the foot, through which muscle fibres run in all directions. The foot of a gastropod is a flat structure used for crawling. Waves of muscular contraction travel along its length, moving the animal slowly over the ground. The foot of a bivalve mollusk is a bulbous or tonguelike organ that is used for burrowing in sand or mud. The foot pushes down into the substrate, swells to anchor itself, and then pulls the rest of the animal down behind it.

In addition to the muscles of the foot, gastropod and bivalve mollusks have large muscles attached to their shells. The columellar (shell) muscles of gastropods pull the foot and other parts of the body into the shell. The adductor muscles of bivalves (Figure 4) shorten to close the shell or relax to allow the shell to spring open, enabling the mollusk to extend its foot or to feed. The adductor muscle can shorten rapidly and close the shell quickly. The muscle is also capable of maintaining the tension needed to hold the shell shut against the spring action of the hinge ligament without using much metabolic energy. Economy of energy is particularly important if the shell has to be kept closed for long periods—for example, for several hours while the mollusk is exposed on the beach at low tide. Fast muscles can shorten rapidly because their cross bridges detach and reattach quickly; however, they use much energy while maintaining tension because there is an energy cost every time a cross bridge detaches and reattaches. Muscles that are economical in their energy usage are generally slow. Accordingly, most bivalve mollusks have two parts to their adductor muscles: a translucent part, which is fast, and an opaque part, which is slow but economical.

Squids and other cephalopod mollusks also swim by jet propulsion. They draw water into the mantle cavity (the cavity that houses the gills) and expel it rapidly. Vigorous movements of this kind provide jet propulsion, but gentler ones serve for breathing by circulating water, and thus oxygen, through the mantle and gills. Fast-swimming squid have mantle cavities whose muscular walls make up as much as 35 percent of the mass of the body.

These walls mainly consist of circular muscle fibres that squeeze water out of the mantle cavity when they contract. Other fibres run radially through the thickness of the wall. These fibres make the wall thinner when they contract, stretching the circular muscle and enlarging the cavity again. Cephalopods do not have longitudinal muscle fibres; however, layers of collagen fibres on the outer and inner surfaces of the muscle prevent the animal from lengthening when the muscles contract. Thus, the circular and radial muscle fibres are antagonistic. Enlargement of the cavity, however, is not solely due to the radial muscle fibres; the cavity tends to expand by elastic recoil of the tissues when the circular muscles relax.

Though many mollusks have shells, most molluscan muscle systems depend on the principle of the hydrostatic skeleton. In some cases, body fluids are involved; for example, the feet of clams are extended and inflated by the inflow of blood. In other cases the muscle itself serves as the incompressible element that must thicken as it shortens or become slender as it elongates, to maintain constant volume. Examples include the shell muscle of the abalone and the tentacles of squid, which are shortened by contraction of longitudinal muscle fibres and lengthened by circular and transverse ones.